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Re: Fractional Pixels Origin? [message #47550 is a reply to message #47549]

Thu, 16 February 2006 14:48

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Wed, 15 Feb 2006 21:31:13 +0000, Wayne Landsman wrote:

>
> "CJCrockett" <ccrockett@astro.umd.edu> wrote in message
> news:1140036794.188403.185210@g14g2000cwa.googlegroups.com.. .
>> A quick question. Does anyone know, definitively, what origin IDL uses
>> when defining fractional pixels? Is (0.0,0.0) the center, bottom left,
>> or other, of the pixel?
>>
>>
> As David said, this is a convention which is set outside of IDL, but
> seeing that you have a "astro" E-mail address I'll say that the FITS
> convention is that [0,0] locates the center of the pixel. (This differs
> from most other image processing standards where [0.,0.] defines the lower
> lefthand corner.)

There are actually 3 conventions in common use, with the center of the
lower-left pixel at [0.0,0.0],[0.5,0.5], and [1.0,1.0]. In the first
case, you have negative fractional pixels as valid. In the last case,
[0.,0.] isn't valid.

Only the middle case gets it right in my opinion. I call this the
"ruler convention". I.e. if you had a ruler marked in pixels and
fractional pixels, you would lay it down on the screen or page, and
could directly read out the fractional pixel location.

FITS does indeed use [0.0,0.0]. I'd urge those of you making the
choice for your programs to save the world confusion, and adopt the
"natural" choice: pixels centered on [a.5,b.5].

JD

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[Message index]

		Re: Fractional Pixels Origin? By: David Fanning on Thu, 16 February 2006 21:15
		Re: Fractional Pixels Origin? By: Greg Hennessy on Thu, 16 February 2006 20:20
		Re: Fractional Pixels Origin? By: David Fanning on Thu, 16 February 2006 16:02
		Re: Fractional Pixels Origin? By: JD Smith on Thu, 16 February 2006 14:48
		Re: Fractional Pixels Origin? By: rm on Thu, 16 February 2006 06:24
		Re: Fractional Pixels Origin? By: Craig Markwardt on Wed, 15 February 2006 14:28
		Re: Fractional Pixels Origin? By: Wayne Landsman on Wed, 15 February 2006 13:31
		Re: Fractional Pixels Origin? By: David Fanning on Wed, 15 February 2006 13:03
		Re: Fractional Pixels Origin? By: JD Smith on Fri, 17 February 2006 13:25
		Re: Fractional Pixels Origin? By: mmiller3 on Fri, 17 February 2006 05:36
		Re: Fractional Pixels Origin? By: Greg Hennessy on Fri, 17 February 2006 05:06
		Re: Fractional Pixels Origin? By: Wayne Landsman on Thu, 16 February 2006 23:50

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